Solar battery cells are semiconductor elements converting light energy to electric power and include a p-n junction type, a pin type, a Schottky type and so on, among which the p-n junction type is especially in wide use. Further, when solar batteries are classified based on a substrate material, they are roughly classified into three kinds, that is, a silicon crystal-based solar battery, an amorphous silicon-based solar battery, and a compound semiconductor-based solar battery. The silicon crystal-based solar batteries are further classified into a single crystal solar battery and a polycrystalline solar battery. Since a silicon crystal substrate for solar batteries can be relatively easily manufactured, the silicon crystal-based solar battery is most widely used.
A demand for solar batteries has recently been increased as a clean energy source, and accordingly, a demand for solar battery cells has also been increased. Further, in view of energy efficiency, it is desired that solar battery cells have as high conversion efficiency from light energy to electric power (hereinafter, also simply referred to as “conversion efficiency”) as possible. As one of the solar battery cells, there is a double-sided light-receiving solar battery cell. The double-sided light-receiving solar battery cell can generate power by absorbing scattered light and reflected light through a cell back surface and thus improves in conversion efficiency as compared to that of a conventional single-sided light-receiving solar battery cell, but is demanded to further improve in conversion efficiency.
In the double-sided light-receiving solar battery cell, an n-type substrate is used, and thermal diffusion using boron as a diffusion source is used when forming a p+ layer in some cases. One of factors inhibiting the improvement in conversion efficiency of the double-sided light-receiving solar battery cell can be the fact that a boron silicide layer remains on a boron diffusion layer on a silicon substrate front surface in a manufacturing process of the solar battery cell. The boron silicide layer is formed when thermally diffusing boron on the silicon substrate.
Generally, the boron silicide layer formed on the silicon substrate is removed by once oxidizing the boron silicide layer to alter it to a boron silicate glass layer and then performing wet etching with a chemical such as a hydrofluoric acid (Patent Documents 1 to 3).